In modern wireless communication systems, antennas are used in a wide variety of electronic devices, such as mobile phones, personal digital assistant (PDA), wireless routers, hand-held tablets, laptops, etc., for transmitting and receiving radio signals. Depending on application, antennas may transmit and receive radio waves at various frequency bands. For example, mobile phones may use antennas to realize wireless communications with base stations at specific cellular frequencies such as 850 megahertz (MHz), 900 MHz, 1800 MHz, and 1900 MHz. Wireless routers, cellular phones may use antennas to communicate at Wi-Fi frequencies such as 2400 MHz and 5000 MHz. In fact, more and more functionalities (e.g., global positioning system (GPS), wireless local area networks (Wi-Fi), Bluetooth, cellular communication, etc.) are now being integrated into a single portable electronic device such as a smartphone. As a result, the number of frequency bands needed to incorporate into a single device is ever increasing. On the other hand, the size of portable electronic devices is fixed or reducing, which in turn imposes strict limitations on the available space where one or more antennas may be housed. Therefore, it is desirable for antenna designers to provide improved antenna structures which utilize the limited antenna space more efficiently.